Chromium in the form of Chromate (CrO4−2) is a widely employed and highly effective corrosion preventive pigment. It is the de facto standard for corrosion prevention in primers applied to aluminum, particularly aerospace aluminum stock (2024-T3, and certain 7000 series alloys for instance). In addition, hexavalent chromate in the form of Strontium Chromate is the benchmark for corrosion prevention in coil coating primers (whether based on Epoxy-Melamine, Polyester or other binder systems) when applied to Zinc, Zinc-Aluminum or similar galvanized or electro-galvanized coil stock.
There has been extensive activity to eliminate hexavalent chromium from these primer systems, which efforts are driven by two related impeti. First, Hexavalent Chromium is toxic; thus, efforts to identify a performance-equal substitute to Strontium Chromate are easily understood. Secondly, there are directives promoted in Europe that require a limit on Chromium content, such that a manufactured article may more readily be recycled.
For the foregoing reasons, there have been numerous academic research projects (see Progress in Organic Coatings, 47 (2003):174-182, Buchheit et al.), and an untold number of industry product development exercises devoted to finding a replacement for Strontium Chromate. There is also a substantial patent literature history describing some of these efforts by industry.
In almost every case examined however, there appears to be one or more flaws in the touted benefits of the described compounds. Typically, the claimed chromate-free inhibitor is in fact not demonstrated in a coating system, but rather in a primer alone. This may disguise a performance flaw, e.g., that the inhibitive pigment leads to blistering. Sometimes, the touted composition is offered based on data for a coating system that is applied only to ferrous metal surfaces. Though it may be free of chromium, the anti-corrosive pigment provides insufficient electrochemical over-potential to prevent corrosion of highly reactive metals such as Aluminum or Zinc. Finally, it is often the case that the benefit of a chrome-free anti-corrosive pigment is made on the basis of unrealistically short accelerated testing; which is deemed unreliable by those skilled in the art. Indeed, many patented offerings, when independently tested, fail to deliver the long duration of outstanding corrosion prevention expected of Strontium Chromate in benchmark accelerated tests.
The present invention overcomes these deficiencies in previous chrome-free anti-corrosive pigments. It provides proof that the inventive compositions are able to deliver performance on a par with Strontium Chromate when evaluated directly alongside the same in realistic coating systems, appropriate for the substrate and end-use application. The present invention does use a ratio of elements in its composition that has been previously disclosed. It differs from the prior disclosed systems in that this ratio of elements is used to make a highly crystalline form of an analog of Hydrotalcite, which contains therein, as a charge balancing ion, a reservoir of the corrosion preventive anion decavanadate. This is considered a Hydrotalcite analog as the other components of the pillared clay are made using Zinc and Aluminum hydroxides, rather than as in natural hydrotalcite Magnesium and Aluminum hydroxides. Previously disclosed were compounds that included the Zinc and Aluminum and Vanadium elements in an approximate range of composition Zn6Al2V10O28(OH)n to Zn6Al2V10O28(OH)y. However, these compositions were all very amorphous, and contained substantial residual co-salts resulting from their means of synthesis. These salts (contaminants) lead to failure by blistering when such amorphous materials are used in full coating systems. Thus, it is not surprising that almost all the literature data describing these materials focuses on their use in primers alone.
The present invention provides a means to avoid the use of salts in the manufacture of such Zinc-Hydrotalcite compositions. As an outgrowth of this approach, the resulting compounds are highly crystalline in nature. Furthermore, due to the lack of tracer salt contaminants, their resistance to blistering in typical coating systems is greatly improved.